Photosystem II contains two symmetry related redox active tyrosine residues, YD(D2TyrI60) and YZ(DI-Tyrl6l). YZ is thought to serve as an electron transfer intermediate between the tetranuclear Mn cluster, where water oxidation occurs, and the photo-oxidized chlorophyl moiety P680. The role of the YD tyrosyl radical is currently unknown; however, it is believed to be involved in protein assembly and stabilization of the Mn cluster. We recently reported the high frequency (139.5 GHz) EPR spectrum of the YD tyrosyl radical of photosystem H. A rhombic powder pattern with principal g values gl=2.00782, 92=2.00450, and 93=2.00232 and hyperfine structure on all three turning points was measured. The high frequency EPR spectrum was acquired under conditions of saturation in which the dispersion signal is detected. Proper replication of the spectral line shapes is only achieved in [unreadable]simulations which account for the line shapes characteristic of saturated dispersion signals. The shift in g, from the free electron g-value has been shown to be sensitive to hydrogen bonding at the phenyl oxygen of the tyrosyl radical. The value of g 1 for YD is consistent with a well-defined hydrogen bond. Density functional calculations indicate that the decreased downfield shift in g I from the free electron g value with increasing hydrogen bond strength arises from both a decreased spin density on the phenyl oxygen -- 5-30% over a range of reasonable hydrogen bond distances (2.0-1.1 A) -- and an increased splitting between ground state and excited state singly occupied molecular orbitals.